A Karman vortex flowmeter to which the present invention pertains has been already proposed by the present applicant in Japanese Patent Application No. 90813/1981 and is shown in FIGS. 1 and 2. FIG. 1 is a front elevation of this known Karman vortex flowmeter, and FIG. 2 is a view for illustrating the construction of the vortex detecting apparatus of the flowmeter.
Referring to FIG. 1, there are shown a pipe line 1, a vortex generator 2 for generating Karman's vortex streets, openings 3 and 3', a vortex detector 4, and optical fiber 5, and a processing circuit 6 for processing the signal detected. The vortex detecting apparatus is comprised of the devices 4 through 6. As shown in FIG. 2, the vortex detector 4 is provided with a vibration chamber 43 having a substantially isosceles triangular cross section, and a vibrating plate 44, which is caused to vibrate by vortices occurring near the vortex generator 2, is installed within the vibration chamber 43. Pressures of Karman's vortices are introduced through openings 41 and 42. The signal processing circuit 6 consists of a light emitting device 6a, a light receiving device 6b and a waveform shaping circuit 6c.
In the operation of my earlier flowmeter, when Karman's vortex streets are generated in the vicinities of the opposite sides of the vortex generator 2 installed in position within the pipe line 1, pressures resulting from the vortices are transmitted through the openings 3, 41 or 42 to the vibrating plate 44 thus to displace it. These vortices occur alternately in the vicinities of the opposite sides of the vortex generator 2 and cause the plate 44 to vibrate. Light from the light emitting device 6a in the signal processing circuit 6 is introduced at the vibrating plate 44 through optical fiber 5a and reflected at the surface of the vibrating plate, and then it is transmitted to the light receiving device 6b through optical fiber 5b. Since the edge surfaces of the optical fibers 5a and 5b are opposed substantially vertically to the vibrating surface of the plate 44 in this construction, the quantity of light entering the light emitting device 6b varies in response to the displacement of the vibrating plate 44. As such, the light receiving device 6b receives a signal corresponding to one cycle of the vibrating plate 44, thus permitting detection of the vibration frequency of the vortices.
Generally, flowmeters of this kind have the disadvantage that generation of vortices becomes unstable and weak vortices are produced at lower flow velocities, thus rendering accurate measurement difficult.
First, the problems of vortex generators will be discussed. FIG. 3 schematically shows one example of prior art flowmeter, in which two pillars K.sub.1 and K.sub.2 are disposed along the flow. Pillar K.sub.1 on the upstream side for producing vortices is triangular in shape, while pillar K.sub.2 on the downstream side is shaped like a plate. This triangular pillar forms a contour resembling a streamline relative to the flow, and therefore the shape is advantageous in that it does not introduce a large pressure loss. However, it cannot produce vortices with ease, and so when the flow velocity is low, measurement is difficult. Also, in such a construction, detection of vortices is carried out on the downstream side of the pillars and accordingly, if pulses or the like occur in the pipe line, variation in flow velocity or pressure produces noise, whereby making accurate detection of vortices impossible.
Also, another flowmeter is known in which a pillar having a substantially isosceles trapezoidal cross section is so disposed that the base is vertical to the direction of flow in order to produce vortices over a relatively wide range of velocity of flow. Unfortunately, this form is disadvantageous in that a large pressure loss results, because the surface against which the flow collides is flat. Still another prior art form of vortex generator has a plurality of pillars which are disposed at regular intervals along the flow for producing vortices. This is however, disadvantageous in that it is complex in construction and expensive to fabricate.
Next, the problems of vortex detectors are discussed. Generally, in a flowmeter of this kind, Karman's vortex streets occurring on the downstream side of a pillar or pillars are very feeble when the velocity of flow is low. Hence, a highly sensitive detector is required. Means using highly sensitive heated wires or ultrasonic waves have the disadvantage that they electrically amplify minute analog signals and so the temperature characteristics and the stability of the detector or detecting circuit considerably affect the measuring accuracy and range. Accordingly, for detectors used for detection of vortices when the flow rate is low it is required that they be relatively unaffected by these factors, and furthermore that they be highly sensitive.
Of these prior art apparatuses, an example of apparatus in which a vibrating plate is displaced by vortex pressure to facilitate signal processings is disclosed in Japanese Utility Model Laid-Open No. 21501/1971, where a vibration chamber is provided within a vortex generator and a vibrator consisting of a plate body is installed on the wall of the chamber, one end of the vibrator being fixed. The velocity or quantity of flow is derived from the vibration frequency of the vibrator. This apparatus is advantageous in that it is simple in construction, because pressure change resulting from occurrence of vortices is directly detected as a displacement or force. However, as the vibrator generates bending vibrations with its one end fixed, it frequently malfunctions due to external vibrations. Particularly when the flow velocity is low, the pressure change resulting from occurrence of vortices is quite small and so it is impossible to discriminate vibrations due to vortices for external vibrations, thereby making accurate detection of vortex frequency unfeasible.
Of conventional vortex detectors, a relatively sensitive vortex detector has a vibrating member which consists of a plate member of light resin and is held so that it can rotate about a revolving shaft. This vortex detector is disadvantageous in that when flow velocity is high and vortex pressure also high, an excessively large displacement or force is applied, because the plate member is displaced in proportion to the vortex pressure. As a result, it will be impossible to detect vortices with accuracy, or the detector may be damaged. An example of an apparatus in which a plate member is bent to detect the distorsion is disclosed in Japanese Patent Laid-Open No. 36933/1980. This apparatus is able to detect vortices stably even when the flow velocity is high, but it cannot detect vortices when the flow velocity is low, because it makes use of bending or flexural vibration of the plate member.
Next, the problems of circuits for processing detected signals will be discussed. DC component E.sub.o (FIG. 26 described later) corresponding to a given quantity of light which is obtained when a vibrating plate is at rest and at a position of equilibrium is added to AC component "a" proportional to change in light quantity caused by vibration of the vibrating plate. The resultant composite signal is detected by light receiving device 6 (FIG. 2). Conventionally, in order to shape the waveform of such a signal into a desired form, a method has been proposed in which a signal is simply amplified in an alternating manner. In this case, frequency of vortices varies widely from 10 Hz to 1 KHz. Generally, this method is also disadvantageous in that it requires a complex circuit configuration. Further, a circuit in which the output signal from a vortex detector is compared with a given set value has also been suggested for shaping the waveform into a desired form. As the light quantity is varied because of contamination of the optics, the DC components of the aforementioned output signal are also varied, but the given set value is unable to follow this change. Hence, it will be impossible to detect the vortex frequency accurately.
Karman vortex flowmeters are also useful for measurement of the quantity of air inhaled by an internal combustion engine of an automobile or the like. However, conventional Karman vortex flowmeters have disadvantages as follows. In nearly all engines of recent vintages of automobiles, oil vapor or leak gas (which is also called "blowby" gas) produced within the crank case is returned to the inlet passage through an air cleaner or the like for preventing air pollution. As such, if a flowmeter is placed in the inlet passage, the optics will be contaminated after long usage, resulting in decrease in the detection sensitivity.
A flowmeter used in the engine of an automobile or the like is usually readily affected by temperature or electrical noise within the engine chamber. In the past, as disclosed in Japanese Utility Model Laid-Open No. 28998/1980, an attempt has been made at cooling the electric circuit of a flowmeter used for detecting vortices with inhaled air to eliminate the effects of temperature. However, when less air is inhaled as during idle running, the cooling effect is lowered and the temperature rise becomes great, thereby reducing the reliability of the electric circuit. Further, means to compensate for the temperature rise become costly. In contrast with this, an arrangement in which the electric circuit for the detection of vortices is installed at a location, for example the passenger compartment, which is superior to the engine chamber in conditions, such as temperature, has been suggested. However, in the course of transmission of a detected feeble electrical signal to the aforesaid electric circuit, the signal is much affected by electrical noise such as that resulting from ignition, thus making measurement impossible.
In a flowmeter as disclosed in the above-cited Japanese Utility Model Laid-Open No. 28998/1980, a processing circuit for processing a detected signal is housed in a bypass passage in order to cool the circuit with air flowing through the bypass passage. In practice, when little air is inhaled, the signal processing circuit can become hotter, but at that time only a minute quantity of air flows through the bypass passage, because it has a greater resistance than the pipe line. Thus, the cooling effect is lower.
It is an object of the present invention to provide a Karman vortex flowmeter equipped with a vortex generator which can generate relatively strong and regular vortices even when flow velocity is low, produces a relatively small pressure loss, and is less affected by disturbances in fluid.
It is another object of the present invention to provide a Karman vortex flowmeter including a detector which is unaffected by external vibrations even at low flow velocities and yet is able to precisely detect only vibrations resulting from vortices.
It is a further object of the present invention to provide a Karman vortex flowmeter capable of detecting vortices stably over a wide range of flow velocity.
It is an even further object of the present invention to provide a Karman vortex flowmeter which can detect vortices with certainty even when a transient phenomenon takes place.
It is a still further object of the present invention to provide a signal processing circuit which is unaffected by a change in light quantity resulting from the variations of light receiving devices and is capable of detecting vortex signals with certainty.
It is a further object of the present invention to provide a Karman vortex flowmeter which is useful for measurement of the quantity of air inhaled by an internal combustion engine of an automobile or the like and is capable of preventing the contamination of the optics in a simple manner.
It is an additional object of the present invention to provide a Karman vortex flowmeter which is useful for measurement of the quantity of air inhaled by an internal combustion engine of an automobile or the like and is less affected by temperature and electrical noise.
It is another object of the present invention to provide a Karman vortex flowmeter which is useful for measurement of the quantity of air inhaled by an internal combustion engine of an automobile or the like and has a sufficient cooling effect even when little air is inhaled and the air becomes hot.
These and other objects and features of the present invention will become more apparent in the course of the following detailed description of the preferred embodiments of the invention taken in connection with the accompanying drawings.